When a fault occurs in power grids in which wind turbines are interconnected, the grid voltage momentarily drops. In this case, at the power grid side, the grid in which the fault has occurred is disconnected and only the unaffected grids are reactivated. This takes about 3 seconds at most.
When the grid voltage drops, the following events occur in wind turbines.
1) Because generator output momentarily drops, this causes overspeed.
2) A large transient current flows in a generator.
3) A large transient current flows also in electric motors of auxiliary units.
In order to safely operate wind turbines to cope with these events, for example, techniques disclosed in Non Patent Citations 1 and 2 have been proposed.
For example, in order to suppress the occurrence of overspeed, Non Patent Citation 1 discloses the use of blade pitch control (line 6 of page 180 and line 14 of page 181 in Non Patent Citation 1).
In addition, in order to avoid possible damage to a converter that controls the rotor current by making a large transient current flow in a rotor coil of a generator, Non Patent Citation 1 discloses a technique in which the converter is temporarily stopped. More specifically, in the disclosed technique, the converter is switched off when a current of a predetermined value or greater flows in the generator rotor coil. Non Patent Citation 1 discloses a technique in which, when the above-described events occur, the rotor coil is short circuited to cause the same operation as a normal induction generator, thereby causing converter control capacity to be lost.
Furthermore, at the time of a voltage drop, the disclosure of Non Patent Citation 2 provides a supply of power to a drive circuit, etc. from an uninterruptible power supply (UPS: Uninterruptible Power Supply).    Non Patent Citation 1: Vladislav Akhmatov, “Variable-speed Wind Turbines with Doubly-fed Induction Generators Part II: Power grid Stability”, Wind Engineering Vol. 26, No. 3, 2002, pp. 171-188.    Non Patent Citation 2: Eric M. Sisa, “Power Outages And Power Dip Ride-Through”, IEEE Annual Texile, Fiber and Film Industry Conference, 1995, pp. 37-43.